1. Field of the Invention
This invention relates to a curing accelerator, an epoxy resin composition and a semiconductor device, and more particularly to a curing accelerator which is suitable for thermosetting resin compositions, an epoxy resin composition containing the curing accelerator, which has excellent curability, storage stability and fluidity and is suitable for resin materials for electric and electronic parts or devices, and a semiconductor device having excellent solder cracking resistance and moisture resistance reliability.
2. Prior Art
As for a method for encapsulating semiconductor elements such as IC, LSI or the like to manufacture semiconductor devices, transfer molding using epoxy resin compositions is widely used on the ground that such a molding method is suited to low-cost mass production. Further, in order to improve properties and reliability of the semiconductor devices, epoxy resins and phenolic resins which serve as a curing agent are also required to be improved.
Meanwhile, in recent years, in the field of electronic equipment, a trend that requires compact design, light weight and high performance is still continued. In accordance with such a trend, degree of integration of semiconductor elements is increased year-by-year, and surface mounting technique for semiconductor devices is also improved. In such a trend, requirements for the epoxy resin compositions used for encapsulating semiconductor elements are becoming increasingly rigorous. Therefore, a problem arises in that the conventional epoxy resin compositions can not satisfy such rigorous requirements.
Specifically, materials used for encapsulating semiconductor elements are currently required to have improved rapid curability for realizing productivity gains as well as improved storage stability for making handling easy in transport and storage.
In general, to the epoxy resin composition for use as a resin material for electric or electronic parts or devices, an adduct of tertiary phosphine with quinones (see, e.g. JP-A-10-25335) is added as a curing accelerator for the purpose of promoting the curing reaction of resins.
However, in such a curing accelerator, a temperature region that exhibits an effect of promoting the curing reaction extends to a relatively low temperature. Therefore, even when an uncured epoxy resin and other components are simply mixed to prepare an epoxy resin composition, the curing reaction of the epoxy resin composition partially proceeds due to heat generated in the system or heat added from an external source. Also, even after mixing is completed, the curing reaction further proceeds while the epoxy resin composition is being stored at room temperature.
Such partial proceedings of curing leads to increased viscosity or lowered fluidity of the epoxy resin composition in a case where the epoxy resin composition is in a liquid form, and in a case where the epoxy resin composition is in a solid form, it leads to the development of viscosity therein. However, since such changes in the condition of the epoxy resin composition do not evenly occur in a strict sense, variations occur in curability of the epoxy resin composition from part to part.
Further, when such a partially cured epoxy resin composition is molded by curing it at high temperature (the word “mold” also includes the meaning of the word “form”, and hereinafter “mold” is used in such a sense), troubles occur in the molding process due to its lowered fluidity, and as a result, a molded product has lowered mechanical, electrical or chemical properties.
Therefore, in a case where a curing accelerator having the possibility of lowering the storage stability of the epoxy resin composition is used, strict control of molding conditions as well as strict quality control in mixing components, and storage and transport at low temperature must be carried out. That is, handling of the epoxy resin composition containing such a curing accelerator is very delicate.
Further, another problem arises in that the solder cracking resistance of a semiconductor device encapsulated with such an epoxy resin composition is not sufficient to withstand surface mount soldering so that the moisture resistance reliability thereof is lowered. The reason is as follows. Specifically, since the semiconductor device is suddenly exposed to high temperature of 200° C. or more when it is immersed in solder or subjected to the reflow soldering process, if adhesion at an interface between a cured product of the epoxy resin composition and base components such as semiconductor elements, a lead frame and the like which are provided inside the semiconductor device is not sufficient, delamination occurs at the interface. The delamination will give rise to cracking in the semiconductor device and lower the moisture resistance reliability of the semiconductor device. Further, if the epoxy resin composition contains volatile components, cracking becomes liable to occur in the semiconductor device due to stress to be generated when the volatile components are explosively vaporized.
However, at the present, semiconductor devices manufactured by encapsulating semiconductor elements or the like with a cured product of the epoxy resin composition containing the curing accelerator described above (an adduct of tertiary phosphine with quinones) do not have sufficient solder cracking resistance and moisture resistance reliability.
Further, to improve the storage stability of the epoxy resin composition, researches on curing accelerators (so-called latent curing accelerators) which suppress changes with age in the viscosity and fluidity of the epoxy resin composition at low temperature and promote curing reaction only when heat is applied during forming or molding have been actively made. Through the researches, it is found that such a latent curing accelerator can be obtained by, for example, protecting active sites of a curing accelerator with ion pairs. As a result, various latent curing accelerators having a structure of a salt of organic acid with phosphonium ion are known (see, e.g. JP-A-2001-98053 (page 5)).
However, semiconductor devices manufactured using such a phosphonium salt as a curing accelerator also do not have sufficient solder cracking resistance and moisture resistance reliability that can withstand surface mount soldering.
Also, as another type of the latent curing accelerator, various latent curing accelerators having a structure of phosphonium-betaine salt have been proposed (see, e.g. U.S. Pat. No. 4,171,420 (pages 2 to 4)), which are intended to enable semiconductor devices to have sufficient solder cracking resistance and moisture resistance reliability that can withstand surface mount soldering. However, a problem also exists with such a latent curing accelerator in that it can not sufficiently cure a semiconductor encapsulating material currently used which contains low molecular weight epoxy resins and phenol aralkyl resins which serve as a curing agent.